In the field of infrared detectors, the use of devices designed in the form of an array which are capable of operating at ambient temperature, i.e. which do not require cooling down to very low temperatures, is known—these are contrasted with detection devices referred to as “quantum detectors” which can only operate at very low temperature, typically at the temperature of liquid nitrogen.
These uncooled detectors traditionally use the variation in a physical unit of an appropriate material as a function of temperature at around 300 K. In the case of bolometric detectors, this physical unit is electrical resistivity.
Such an uncooled detector is generally associated with:                means of absorbing the infrared radiation and converting it into heat;        means of thermally insulating the detector so that its temperature can rise due to the effect of the infrared radiation to be detected;        thermometric means which, in the context of a bolometric detector, use a resistance element;        and means of reading electrical signals provided by the thermometric means.        
Detectors intended for infrared imaging are conventionally produced as a one- or two-dimensional array of elementary detectors, said array being “monolithically” formed or mounted on a substrate generally made of silicon which incorporates means of sequentially addressing the elementary detectors and means of electrically exciting (stimulating) and of pre-processing the electrical signals generated by these elementary detectors. These means of sequential addressing, electrical excitation and pre-processing are formed on the substrate and constitute a read circuit. The term “read” denotes the formation of an electrical signal on the basis of the state of the sensing elements.
In order to obtain a scene using this detector, the scene is projected through suitable optics onto the array of elementary detectors, each of them constituting an image dot or pixel, and clocked electrical stimuli are applied via the read circuit to each of the elementary detectors or to each row of such detectors in order to obtain an electrical signal which is an image of the temperature reached by each of said elementary detectors. This signal is then processed to a greater or lesser extent by the read circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
The essential difficulty encountered when using bolometric detectors is the extremely small relative variation in their electrical resistance which is representative of the local temperature variations in an observed scene relative to the average value of these resistances.
The constructionally dictated presence of a finite thermal resistance between the bolometer and the substrate means that the temperature of the bolometer is influenced by the temperature of the substrate much more sensitively than temperature variations due to the incident flux which are the only variations to be taken into account from the point of view of the signal to be detected. Residual fluctuations in the temperature of the substrate under normal thermal stabilization conditions, all the more so if the detector does not have such a thermal stabilization system as is increasingly the case with this type of detector in order to reduce its cost, consequently produce an unwanted component in the signal obtained from the bolometer which adversely affects the quality of the signal. Conventionally, the substrate isothermally controlled in order to prevent or at least limit this effect.
In addition, “compensation” structures are used in order to minimize the effects of the temperature of the focal plane on the detector's response. These structures, which are usually bolometers referred to as “blind bolometers”, i.e. bolometers which are not sensitive to the incident optical flux but which are sensitive to the temperature of the substrate, are used in order to generate a so-called compensation current which is subtracted from the current obtained from the imaging bolometers, i.e. the detection bolometers, due to the way in which the electronic circuit is configured.
These compensation structures are typically built so that they have a very low thermal resistance, particularly negligible thermal resistance, relative to the substrate, unlike the imaging bolometers.
This way, most of the current referred to as “common-mode current”, i.e. current which is not representative of information originating from the scene to be detected, is eliminated.
Also and advantageously, because the compensation structures are substantially at the same temperature as the read circuit and therefore the focal plane, this actually allows significant rejection of any fluctuations in the temperature of the focal plane. Arranging these compensation structures “identically” and repetitively in each column of the array so as to reduce the complexity and overall dimensions of the circuit is a known tactic.
Every bolometer column is sequentially compensated by the same compensation structure when the image is electronically scanned one row at a time. However, compensation structures naturally exhibit spatial variations in resistance because of the technology processes used in their fabrication (which normally originate from the semiconductor industry).
In addition, blind bolometers, like imaging bolometers, as well as certain functions of the read circuit, are affected by noise phenomena in general and so-called “1/f” noise in particular. 1/f noise typically produces low-frequency drift, especially very low frequency drift, of the output level of the sensors which adversely affects the quality of the imager. The columnar arrangement of the compensation structures has a negative impact on the quality of the image because of low-frequency variations in the compensated signal which are asynchronous from one column to the next. Besides any special design and implementation measures taken in order to reduce this variability, compensation algorithms must, generally speaking, be developed and applied at the output of the imager in order to improve image quality.
Thus, in order to form an image sample (frame), all the bolometers in the first row are addressed (biased) simultaneously and their current is compensated by using the blind compensation structure located at the end of the column. The “row” signal obtained is processed and transferred to the output while the second row is addressed and this process is repeated until the last row is reached in order to complete the frame and then restarts identically for the next frame (see diagram in FIG. 2). It is apparent that low-frequency variations in the compensation current which initially affects all the elements in each column identically but differently from one column to the next, thereby produce slight, slowly-changing columnar contrast which has an adverse effect on the image quality. Besides the design and technology efforts made in order to reduce fixed variability and low-frequency noise, compensation algorithms must be developed and used in order to reduce these effects to a minimum level.
Recent imaging components use algorithmic processes to correct residual drifts in the output signal which are associated with fluctuations in the temperature of the focal plane. This approach aims to do away with thermal control devices (“thermoelectric” modules based on the Peltier-effect) which are expensive in terms of the actual component itself and its associated means of use. These simplified components are commonly referred to as “TECless” (TEC stands for ThermoElectric Cooler). The efficiency of these processes depends significantly on correctly and accurately assessing the variability of these compensation structures over the temperature variation range of the focal plane which one intends to allow for the envisaged application.
Read circuits for resistive bolometric detectors which use blind bolometers are described, for instance, in the following applications:                “Uncooled amorphous silicon technology enhancement for 25 μm pixel pitch achievement”, E. MOTTIN et al, Infrared Technology and Applications XXVIII, SPIE Vol. 4820;        “320×240 uncooled microbolometer 2D array for radiometric and process control applications” B. FIEQUE et al; Optical Systems Design Conference, SPIE 5251, September 29;        “Low cost amorphous silicon based 160×120 uncooled microbolometer 2D array for high volume applications” C. TROUILLEAU et al; Optical Systems Design Conference SPIE 5251-16.        
The electronic structures described in relation to the prior state of the art are designed primarily in order to read bolometers in the active array but can also be used to read blind bolometers if the read timing diagrams are adapted. But these cases are illustrated very schematically in FIG. 2.
The principle of reading an active array of bolometers is explained below in relation to FIG. 1.
Pixel 1 (the term “pixel” is construed here, by extension, as denoting all the structures located so that they are under the influence of one elementary detection point) comprises an active bolometer 2, an NMOS charge injection transistor 3 and a switch 4 which connects pixel 1 to read column 5 and is represented here by a dashed line. Compensation structure 6, which is also referred to as a base clipper in the terminology used in the technical field in question, comprises a blind bolometer 7 connected to power supply VSK and PMOS charge injection transistor 8. During normal operation, the PMOS transistor is in saturation mode. Its current Icomp is defined by the expression:
  Icomp  =      Vcomp    Rcomp  
where:                Vcomp denotes the voltage across the terminals of compensation bolometer 7;        Rcomp denotes the resistance of said compensation bolometer.        
Being the active arm, the current flowing through NMOS charge injection transistor 3 is expressed by the relation:
  Iac  =      Vac    Rac  
where:                Iac denotes the current of the active arm;        Vac denotes the voltage across the terminals of active bolometer 2;        Rac denotes the resistance of said active bolometer.        
The bias voltages of the MOS charge injection transistors are chosen so that, in the absence of any incident scene light flux, i.e. for example when the system is optically shuttered, the difference in current dI=Icomp−Iac between the active arm and the blind arm is substantially zero.
Reading an active bolometer is a two-phase operation. The first phase involves actuating “reset” switch 9 which short-circuits integration capacitance 10 of operational amplifier 11. This gives:Vout=VBUS 
Read column 5 shown by dashed line 5 is therefore brought to the potential VBUS. “Reset” switch 9 is then opened and “select” switch 4 is closed to connect pixel 1 to read column 5. Current difference dl is integrated by capacitance Cint 10 over finite integration time Tint. Integration produces an output voltage level referred to as “continuous level” or NC in the reference case where a uniform temperature scene is observed, this typically reveals the variability of the imaging array. This is the standard method for characterizing the reading of active bolometers.
  NC  =      VBus    -                  Tint        Cint            ⁢      dI      
Bolometers are biased so as to ensure both a dynamic output signal response and efficient compensation.
A more rigorous expression would be obtained by considering, for the last term, the integral of the function dI(t) over Tint because currents Iac and Icomp are not constant. However, for the sake of clarity, the above expression is sufficient to explain the parameters which are to be taken into consideration.
This read system has certain limitations associated with the way in which the columnar compensation pattern is reproduced on the read circuit. In fact, each column has a compensation bolometer and a PMOS charge injection transistor. Imperfect reproduction of these various elements from one column to the next which is inherent in the intrinsic spatial variability of the fabrication technologies used results in non-uniform compensation efficiency. This statistical variability results in a compensation current which is not uniform from one column to the next and causes the appearance of visible columnar contrasts which thus affect the available signal.
The conventional read circuit for active bolometers can be used in order to read blind compensation bolometers. To achieve this, at the end of a frame (i.e. after reading the last row), the first capacitance reset phase is performed. Once again:Vout=Vbus 
“Reset” switch 9 is then released but “select” switch 4 is left open in order to measure the value of the resistance of blind bolometer 7 on its own. All the current which flows in the compensation arm is therefore integrated over time Tint by capacitance Cint 10. In this situation, the output voltage Vout of the system is expressed as follows:
  Vout  =            Vbus      -                        Tint          Cint                ×        Icomp              =          Vbus      -                        Tint          Cint                ·                  Vcomp          Rcomp                    
The conventional read circuit therefore makes it possible to access the resistance value of blind bolometers through the output voltage of the system.
      R    comp    =            Cint              Tint        ×        Vcomp              ×          (              Vbus        -        Vout            )      
This type of circuit associated with this particular implementation has the advantage of providing direct access, for each frame, to the resistance value of the blind compensation bolometer and therefore, in principle, provides the necessary data for the correction algorithm (elimination of columnar image interference). Nevertheless, it does have certain limitations.
Firstly, the system integrates all the bias current of the bolometer. This current is typically 20 to 50 times greater than the current which is normally integrated when reading an active bolometer in the standard compensated mode. Consequently, the integration time must be reduced in comparison with reading a row of active bolometers in order to prevent integration capacitance Cint becoming saturated well before the end of the integration time. This complicates the implementation of reading from the point of view of managing the timing of stimuli.
In addition, this cycle thermally disturbs compensation bolometers (which typically have a very low thermal resistance which is, however, not zero) and therefore their resistance. When the active rows are read, the compensation bolometers are biased periodically (depending on the row frequency) for a constant time span. The Joule effect, linked with the flow of compensation current through these bolometers, brings them to an identical temperature at the start of every integration cycle (for each row) or, more precisely, subjects them to a precisely repetitive thermal profile during every integration cycle until the last row is read. When the row of blind compensation bolometers is then read, this cycle is disturbed because one must apply an integration time Tint which is much shorter (by a factor of 20 to 50 as explained above). This results in temporary thermal disturbance which causes a continuous-level shift and hence, given any scene, image disturbance at the start of the next frame.
Interference with the same origin also occurs if the frame is scanned for a duration which is less than the image refresh time. There is a latency time, which may be considerable, between successive frame reads, i.e. between the end of one frame read and the start of the next frame read.
This problem can, for example, be resolved by making the read circuit more complex in order to keep the temperature of the compensation bolometers substantially constant or at the least maintain the periodicity of the temperature timing profile over all the rows of the frame. To achieve this, a “preheat” or “substitution” current similar to the average current which flows through the blind bolometers during the integration time can be injected by using a special device in order to maintain the temperature of the blind bolometers during intervals when there are no integration periods. This device typically comprises an additional source of fixed current supplemented by a repetitive switching system with the active arm driven so as to eliminate breaks between periods. This substitution-current switching is activated between two row reads as well as in the time interval between reading the last row (blind in this case) and restarting the first row of the next frame. The temperature and therefore also the resistance of the compensation bolometers are therefore substantially constant over time: there is no longer any transient thermal disturbance.
There is then a problem if the temperature of the component varies and there is no temperature stabilization device (which is often the case). Classically, the nominal operating temperature range of detectors is −40° C. to +120° C. The preheat current source is then sized to give a median operating point, for example 30° C. At this temperature, the substitution current is adjusted so that it is representative of the average current of the active arm. This adjustment becomes inadequate if the temperature of the focal plane differs greatly from this point and one is again confronted with at least some of the thermal variation defects which affect blind bolometers in the read time interval between one frame and the next frame. This problem is also solved by the invention described below.
Another more problematic limitation associated with this circuit/implementation combination is relevant with regard to software (algorithmic) correction of image columning. In fact, the relevant point in terms of algorithmic correction is not the absolute value of the resistance of the blind bolometer but rather its variability, i.e. its distribution from one column to another. If there is precisely zero variation, there is no columning, regardless of the common value of the compensation resistances. From this point of view, the system for reading blind bolometers according to the prior art explained above is not suitable for precisely evaluating differences between resistances. One is interested in the sensitivity of the output voltage as a function of the variation in the resistance of the blind bolometer which is given by the following relation:
            δ      ⁢                          ⁢      Vout              δ      ⁢                          ⁢      Rcomp        =            Tint      Cint        ×          Vcomp              Rcomp        2            
Given the resistance values of blind bolometers and the limitation placed on the integration time discussed above, the value of the sensitivity thus defined is relatively small. By way of a numerical example:                Cint=6 pF        Tint=3 μs        Vcomp=3V        Rcomp=1.6 MΩ        Output voltage swing=3 V        
            δ      ⁢                          ⁢      Vout              δ      ⁢                          ⁢      Rcomp        =            586      ⁢                          ⁢      e        -          9      ⁢                          ⁢      V      ⁢              /            ⁢      Ω      
The gain (this term is equivalent to “sensitivity”) of the structure is relatively small and the wanted signal is all the more difficult to analyze (less accurate). A resistance variation of 3 kΩ actually represents a 2 mV wanted signal. For those skilled in the art, assuming a 12-bit analogue-to-digital converter (ADC) which covers the entire dynamic response (typically 3 V) of the detector, the wanted signal represents 2.7 least significant bits (LSB), i.e. less than 2 bits which can actually be used and this is typically totally inadequate to feed effective algorithmic correction.
The invention proposes a method for precisely evaluating the variation (distribution) in the resistance of these compensation bolometers and any drift thereof in time. The main attraction is to provide algorithm developers with data which is more accurate than that provided by techniques according to the prior art and, consequently, to offer improved corrected image quality.